C++ Null-Object vs Pointer Callback

callback.cpp

#include <iostream>

struct HelloData {
  int i = 0;
  const char* s = "";
};

struct CallbackBase {
  virtual ~CallbackBase() = default;
  virtual void Hello(HelloData const& d) = 0;
};

struct NoopCaller final : CallbackBase {
  virtual void Hello(HelloData const&) override { /*noop*/ }
};

inline NoopCaller Noop;

int Resultify(int val) {
  if (val >= 0 && val <= 42) {
    val *= 42;
  }
  return val;
}

int WithPointer(int val, CallbackBase* pOpt=nullptr) {
  if (pOpt) { pOpt->Hello({ val, "from WithPointer" }); }
  return Resultify(val);
}

double WithObject(int val, CallbackBase& opt=Noop) {
  opt.Hello({ val, "from WithObject" });
  return Resultify(val);
}

// ------ different compilation units ------

struct LogCallback : CallbackBase {
  std::string name = "LogCallback";
  virtual void Hello(HelloData const& d) override { 
    std::cout << "[" << name << "]: " << d.i << " : " << d.s << "\n";
  }
};

int main()
{
  using std::cout;
  cout << "Hello ...!\n";
  LogCallback logger;
  logger.name = "log from main";
  WithPointer(23, &logger);
  WithPointer(24);
  WithObject(42, logger);
  WithObject(43);
}